The present invention relates to a crosslinking agent suitable for use in curing acrylic rubbers.
It is known that acrylic rubbers are superior in heat resistance and oil resistance to general rubbers. Sulfur vulcanization is not applicable to the acrylic rubbers because they have no unsaturated double bond.
Therefore, in order to make crosslinkable, acrylic rubbers have been generally produced by copolymerizing alkyl acrylates with comonomers to impart a crosslinkability, e.g. halogen-containing monomers such as 2-chloroethyl vinyl ether and vinyl chloroacetate, or epoxy-containing monomers such as allyl glycidyl ether, glycidyl acrylate and glycidyl methacrylate, as disclosed in Japanese Patent Publication Kokoku No. 57843/1986. These acrylic rubbers are crosslinkable with a crosslinking agent or curing agent, e.g. a fatty acid soap or an amine compound such as triethylenetetramine or hexamethylenediamine carbamate.
However, these known acrylic rubbers have some defects. The acrylic rubbers prepared by copolymerization with the halogen-containing monomers have the defects that the water resistance is poor because the halogen has a reactivity to water, and that they cause prevulcanization of a mixed compound during roll processing or storage thereof because the reaction of the monomer units introduced to impart a crosslinkability with a crosslinking agent proceeds too fast. The acrylic rubbers prepared by copolymerization with the epoxy-containing monomers have no particularly serious defects, but still have problems that they are inferior in heat resistance to the halogen-containing acrylic rubbers and also are insufficient in water resistance and oil resistance and, therefore, they can not be satisfactorily applied to uses which require particularly superior properties.
There is also proposed by the present inventors an acrylic rubber crosslinkable with a thiuram compound, which is prepared by copolymerizing acrylic acid esters with a cyanoacetic acid compound or a malonic acid compound as a comonomer for imparting a crosslinkability, as disclosed in Japanese Patent Publication Kokoku No. 3770/1979 and No. 28437/1979. This acrylic rubber has good water and oil resistances and also has a heat resistance on the same level as that of conventional acrylic rubbers. However, since this acrylic rubber has been used with a thiuram crosslinking agent which has a relatively low activation energy, it leaves room for improvement of heat resistance.
It is an object of the present invention to provide a curable composition capable of providing an acrylic rubber having excellent water resistance, oil resistance and heat resistance.
Another object of the present invention is to provide a crosslinking agent suitable particularly for use in acrylic rubbers containing units of a cyanoacetic acid monomer or a malonic acid monomer as crosslinking sites, thus giving cured acrylic rubbers having an improved heat resistance as well as excellent water and oil resistances.
These and other objects of the present invention will become apparent from the description hereinafter.